Error reduction in semiconductor processes

ABSTRACT

A photolithography system includes a photolithography tool  32  that includes a stage upon which a semiconductor wafer is mounted. The tool is operable to move the stage to automatically focus a pre-determined image on a surface of the semiconductor wafer. The tool is further operable to log movements of the stage. The system also includes an automation host computer  36  operable to poll the photolithography tool  32  to obtain data reflecting the logged movements of the stage. The automation host computer  36  is further operable to analyze the data and compare the data to pre-determined error conditions. The host computer also takes a pre-determined action, including sending an electronic mail message to the personal computers  38  of relevant line personnel, in the event the data meets the pre-determined error conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of patent application Ser. No.09/972,497, filed Oct. 5, 2001, now U.S. Pat. No. 6,645,684.

BACKGROUND OF THE INVENTION

This invention is in the field of semiconductor processing, and moreparticularly in the field of photolithography.

In semiconductor processing, photolithography is the process of definingparticular features on the surface of a semiconductor wafer. The featureis typically defined with a patterned exposure into a photosensitivematerial that has been previously deposited on the wafer surface. In atypical photolithographic system, a master pattern image in the form ofa photomask or reticle is replicated across the surface of the wafer.The image is typically projected onto the photosensitive material (e.g.photoresist) through a lens system. The quality of the image, and hencethe quality and reliability of the resulting feature on thesemiconductor surface, is directly related to mechanical parameters suchas the spacing and orientation of the lens system relative to the chuckand stage upon which the wafer is placed.

Lithography equipment typically includes a mechanism for fixing thewafer onto a chuck that rests on a movable stage. If the chucking is notdone correctly, or if contamination exists on the chuck or the waferbackside, the wafer will not lie flat on the surface of the chuck. Thisoften results in resolution failures as the system has difficultyfocusing the image in the area of the topographical feature caused bythe chucking error. The local tilt of the wafer surface in the area ofthe undesired topographical feature can cause misformation of theprojected image. The imaging errors can result in the scrapping of oneor more die of the wafer, or indeed one or more die of multiple wafersor multiple lots of wafers if a repeatable and persistent error goesundetected.

Photolithographic tools include a stage that is capable of highlyaccurate and precise positioning so that the wafer surface is inposition to receive a focused projected image. In addition to movementup, down, and side-to-side, a stage must also tilt when necessary toimage a desired feature. FIG. 1 shows a prior art stepper or step andscan photolithographic tool. The stage 10, wafer chuck 12, and wafer 14are moved relative to the reticle 16. Other tool components include thepellicle 18, the lens system 20, and the light source 22. A stepper tooltypically exposes the entire reticle onto the wafer at once, whereas astep and scan tool exposes the image by scanning a slit over the surfaceof the reticle. In either case, the stage must be positioned accuratelyand precisely for proper exposure of the desired feature in thephotoresist. FIG. 2 illustrates the movements of a typical stage.

The photolithographic tool usually achieves focus on the wafer byanalyzing an alignment key such as a plus-shaped mark or bulls-eye onthe wafer. A typical stage is monitored with laser interferometers withresolution on the order of less than one nanometer as the tool seeks toposition the wafer for optimum focus and resolution of the projectedimage. The degree of translation of the stage along x-, y-, and z-axesfor up, down, and side-to-side movements as well as degree of stage tiltas the tool seeks to focus the projected image on the wafer surface areroutinely measured and recorded on a die-by-die basis as the image isreplicated across the wafer.

FIGS. 3 a to 3 d are depictions of light impinging upon the photoresistlayer on a semiconductor wafer. FIG. 3 a is an ideal situation in whichthe light paths striking the photoresist surface all travel equallengths. FIG. 3 b shows the effects of a wafer surface that is inclinedrelative to the light source. FIG. 3 c shows the effects of warpage ofthe wafer or perhaps dishing of the photoresist layer. FIG. 3 d showsthe effects of either a particle on the backside of the wafer or adefect such as a scratch in the photoresist. Note that in the cases ofFIGS. 3 b, 3 c, and 3 d, the light paths are not equal, a fact whichtypically results in poor reproduction of the reticle image in thephotoresist. To counteract this, a typical photolithographic toolattempts to tilt the stage relative to the light source as it steps orscans over the inclined or warped surface, or over the undesirabletopographical feature, as the case may be.

BRIEF SUMMARY OF THE INVENTION

In one embodiment of the invention, a method is disclosed for reducingthe effect of errors in a semiconductor process wherein the processincorporates a photolithography tool in which a stage holding asemiconductor wafer is automatically moved into focus. The methodincludes the steps of collecting data indicating the movement thephotolithographic tool performs to bring the semiconductor wafer intofocus; comparing the data with pre-determined error conditions for themovement; and generating a signal to indicate that the data meets thepre-determined error conditions.

In another embodiment of the invention, a method is disclosed forreducing the effect of errors in a semiconductor process wherein theprocess incorporates a photolithography tool in which a stage holding asemiconductor wafer is tilted during autofocusing in response to atopographical feature on the wafer. The method includes the steps oflogging the mean standard deviation of the pitch of the stage for eachexposure the tool makes on the wafer; comparing the logged mean standarddeviation of the pitch of the stage for each exposure to pre-determinederror conditions; and implementing a pre-determined action in the eventthe logged mean standard deviation of the pitch of the stage meets thepre-determined error conditions.

In still another embodiment of the invention, a photolithography systemis disclosed. The system includes a photolithography tool that includesa stage upon which a semiconductor wafer is mounted. The tool isoperable to move the stage to automatically focus a pre-determined imageon a surface of the semiconductor wafer. The tool is further operable tolog movements of the stage. The system also includes an automation hostcomputer operable to poll the photolithography tool to obtain datareflecting the logged movements of the stage. The automation hostcomputer is further operable to analyze the data and compare the data topre-determined error conditions. The host computer also takes apre-determined action in the event the data meets the pre-determinederror conditions.

An advantage of the invention is that it provides early detection of achuck or wafer contamination or other process error and thus eliminatescosts associated with processing wafers with die that eventually have tobe scrapped as a result of poor resolution caused by the processingerror. The invention is particularly useful for detecting repetitiveerrors that can adversely affect multiple lots of wafers and is fastenough to respond to wafer defects before a wafer or lot of wafers movesfrom the photolithography step to photoresist development.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a side view diagram of a prior art photolithography tool.

FIG. 2 is an illustration of the possible movements of the stage of aphotolithography tool.

FIG. 3 is a diagram showing the adverse effects on image focus due toimperfections in alignment and in the surface shape of a semiconductorwafer.

FIG. 4 is a block diagram of a portion of an embodiment system showingthe relation of an automation host computer to a photolithography tooland to the computers of relevant line personnel.

FIG. 5 is a plot of mean standard deviation of stage pitch for thevarious die on a semiconductor wafer showing no pitch abnormalities orprocess errors.

FIG. 6 is a plot of mean standard deviation of stage pitch for thevarious die on a semiconductor wafer showing abnormalities consistentwith contamination on the backside of the wafer or the mounting chuck.

FIG. 7 is a plot of mean standard deviation of stage pitch for dienumbers one through ten on all of the wafers in a lot in which no pitchabnormalities or process errors are indicated. The plot also shows thepre-determined error limit.

FIG. 8 is a plot of mean standard deviation of stage pitch for dienumbers one through ten on all of the wafers in a lot in whichabnormalities consistent with contamination on the backside of the waferor mounting chuck are present. FIG. 8 also shows an electronic messagethat is sent in response to detection of the abnormalities.

FIG. 9 is an example of a yield enhancement wafer map upon which tiltfocus errors and abnormalities are plotted.

FIG. 10 is a schematic diagram of a model used by an automation hostcomputer to analyze tilt focus data and take appropriate actions whenabnormalities or irregularities appear in that data.

FIG. 11 is a diagram illustrating the actions taken by the automationhost computer upon detecting abnormalities in tilt focus data.

FIG. 12 is a flow chart illustrating the actions taken by the automationhost computer in response to abnormalities in tilt focus data for agiven number of die and wafers in a lot.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 is a block diagram of a portion of a semiconductor integratedcircuit fabrication process. The initial block 30 represents theequipment used to deposit, spin, and bake the photoresist onto a wafer.It is followed by the photolithography tool 32 and then by the equipment34 dedicated to photoresist development. An automation host computer 36is coupled to the photolithography tool and is also coupled to thepersonal computers 38 of relevant line management personnel through alocal area network, for example.

In one embodiment of the invention, data recording the movement of thestage as the tool 32 seeks to maintain focus as it steps or scans areused to detect die-to-die, wafer-to-wafer, and lot-to-lot defects orerrors that have an adverse effect on yield. The automation hostcomputer 36, which controls the tool, reacts to detection of suchproblems by generating an alert signal and by taking an action specifiedin an action look-up table, for example. The severity of the problemdetected through analysis of the stage movement data determines theaction taken by the computer 36 to maintain the best yield in thecircumstances. Actions that the computer 36 can take include noting theproblem in the relevant data log for a particular die, wafer, or lot,and in serious cases, by putting a lot on hold or taking the tooloff-line. The system is also enabled to alert line personnel topotentially yield-reducing problems, typically by sending an electronicmessage to the personal computers 38 of the personnel.

In the preferred embodiment method, stage tilt focus data from ascanning photolithographic tool 32 is used to detect wafer chuckingerrors (e.g. contamination on either the chuck or the wafer backside) orother problems that result in undesirable topography at the wafer orphotoresist surface. The tool 32 logs the mean standard deviation of thestage pitch (hereinafter referred to as the “tilt focus data”) as itscans the reticle to image a particular die. An automation host computer36 polls the scanning tool 32 and pulls this tilt focus data for eachdie for each wafer. The data are then used in a model designed to detectfocus hot-spots on a wafer and within a lot of wafers. Using the model,the host computer 36 determines whether the tilt focus data is withinone of several ranges. The host computer then consults an action look-uptable and implements the appropriate action, whether it be to alertrelevant personnel (e.g. via e-mail), put the lot on hold, or even logthe system down entirely if the problem appears to threaten multiplelots of wafers. The host computer 36 can typically perform its analysisof the tilt focus data for a wafer within one minute, a time span shortenough to allow for action to be taken to hold or stop a wafer or lot ofwafers before the wafer or lot of wafers proceeds to the subsequentprocessing step, which is typically photoresist development 34.

FIG. 5 is a plot of the mean standard deviation of the stage pitch (i.e.the tilt focus data) for each die on a particular wafer. The tilt focusdata for the dies on this particular wafer are all less thanapproximately 10×10⁻⁶ radians. In contrast, the data in FIG. 6 showsseveral die with a stage pitch having a mean standard deviation that issubstantially higher, on the order of 30×10⁻⁶ radians to 90×10⁻⁶radians, which indicates that the stage attempted to adjust to anon-flat wafer surface topography when scanning those dies. FIG. 7 showsthe tilt focus data for dies one through ten for all the wafers in aparticular lot. This plot also includes the predetermined limit of35×10⁻⁶ radians set by process line personnel. All of the dies shown are“good” in that the mean standard deviation of the stage pitch is lessthan the predetermined limit. In contrast, FIG. 8 shows tilt focus datafor a lot containing “bad” dies. In particular, the tilt focus data fordie number eight on several wafers exceed the 35×10⁻⁶ radians criteria.Data appearing in the box 80 in FIG. 8 indicates dies likely to sufferfrom focus abnormalities. In response to the data appearing in the box80, an electronic mail message 82 is automatically generated by theautomation host computer and is sent to the relevant line personnelmonitoring the photolithographic step of the wafer fabrication process.

FIG. 9 is a yield enhancement wafer map showing how the tilt focus datacan be displayed to enable line personnel to easily determine thelocation of a potential source of the undesirable topography, such ascontamination on the backside of the wafer or on the wafer chuck. Arepetition of problems in the same location over several wafers mayindicate that a particle or other contamination may exist on the chuck.It could also indicate that a step upstream in the process may beleaving residue in a certain pattern on the backside of wafers. Earlydetection of this type of problem is of course essential to maintaininghigh yield of the process. In prior art systems, such problems wouldtypically go undetected for an extended period as there was littlefeedback from the process to assist in identifying the source andlocation of focus errors.

FIG. 10 is a logic diagram of the model that is implemented on a toolinterdiction modeling system to allow the automation host computer 36(in FIG. 4) in the system to take appropriate action when tilt focusdata hotspots are detected. The model includes three parts: a die/wafercounter 100, a string builder 102 to keep track of the die and wafercount in the event of a detected failure; and the action look-up tablethat determines the actions to be taken in the event of focus dataabnormalities. The automation host computer is triggered to access theaction look-up table by events 104 in the model.

FIG. 11 is an illustration of the steps taken by the automation hostcomputer in response to the triggering event 104. In this embodiment,after consulting the look-up table, the automation host computer electsto put the lot of wafers on hold. This is indicated in signal panel 110by the event identification “hot_spot_lothold.” The automation hostcomputer then sends the lot-on-hold signal 112, followed by anelectronic mail message 114 to relevant line personnel notifying them ofthe focus data abnormalities.

FIG. 12 is a flow chart illustrating the focus monitoring process. Theprocess begins with a step 120 in which the automation host computerpolls the photolithographic tool and downloads shot log data from thetool for a wafer lot. This data set includes the mean standard deviationof the stage pitch (i.e. tilt focus data that is referred to in FIG. 12as “APMD values”). In the next steps, 122 and 124, the shot log data isput into the model and readied for evaluation. In step 126, the modeltests whether the tilt focus data for a particular die exceeds apre-determined threshold, in this case 35×10⁻⁶ radians. If the tiltfocus data is all less than 35×10⁻⁶ radians, the computer takes noaction (128). However, if any of the data exceeds 35×10⁻⁶ radians, thenumber of die and wafers containing tilt focus data exceeding thatthreshold is evaluated (130). In this embodiment, if less than sixwafers are affected, an electronic mail message is sent to relevant linepersonnel notifying them of the abnormal data, but no other action istaken by the computer other than continued monitoring (132). If morethan two die on a wafer or more than six wafers in a lot are affected byabnormalities, in step 134, the computer adds a comment in the lotrecord and then sends a signal to put the lot on hold at a particularstep in the process. It also sends an electronic mail message torelevant line personnel notifying them of the action, along with a listof the affected die and wafers. The computer then (136) determines thelocation of the affected die on the wafer. Die locations toward thecenter of the wafer and away from the wafer edge are considered “primedie” and problems affecting those die locations are of more concern thanproblems affecting “edge die”. If the computer determines that only edgedie are affected (138), it takes no action. However, if prime die areaffected (140), the computer logs the system down and signals the needfor a test to be run to determine wafer flatness. In addition, anappropriate comment is inserted in the lot record by the computer.

While the present invention has been described according to itspreferred embodiments, it is of course contemplated that modificationsof, and alternatives to, these embodiments, such modifications andalternatives obtaining the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. For example, where stagemovement data are relied upon in the embodiments described herein, oneskilled in the art will appreciate that the movements made by aphotolithography tool to achieve focus on a semiconductor wafer couldinvolve movement of the reticle relative to a stationary stage, oralternatively could involve movement of both the reticle and the stage,in which case the movements of the stage relative to the reticle wouldcould be used in the inventive method regardless of which tool componentmakes the movement. It is contemplated that such modifications andalternatives are within the scope of this invention as subsequentlyclaimed herein.

1. A photolithography system, comprising: a photolithography tool whichincludes a stage upon which a semiconductor wafer is mounted, whereinsaid tool is operable to move said stage to automatically focus apre-determined image on a surface of said semiconductor wafer, said toolfurther operable to log movements of said stage; and an automation hostcomputer operable to poll said photolithography tool to obtain datareflecting said logged movements of said stage, said automation hostcomputer further operable to analyze said data and compare said data topre-determined error conditions and to take a pre-determined action inthe event said data meets said pre-determined error conditions.
 2. Thesystem of claim 1 wherein said data reflecting said logged movements ofsaid stage comprises data indicating the pitch of the stage.
 3. Thesystem of claim 1 wherein said pre-determined action comprises sendingan electronic mail message to personnel monitoring said process.
 4. Thesystem of claim 1 wherein said pre-determined action comprisessuspending an affected wafer or wafers from continued processing.
 5. Thesystem of claim 1 wherein said pre-determined action comprises takingsaid photolithography tool off line.